EDIT: I was wrong and misunderstanding how these options interact, this doesn't solve the stated problem. Still good to have in your config but not as powerful as desired, sorry. Keeping comment contents mostly for posterity's sake.

Under the hood `git pull` is just `git fetch`+`git merge` so these two configs together would ideally enable the behavior requested (but unfortunately don't):

    [fetch]
        all = true
    [pull]
        ff = only

I also disable fast-forwarding with `git merge` since I prefer the branch topology you get without it. This doesn't break natural `git pull` fast-forwarding either, thanks to the above config:

    [merge]
        ff = false

Commands to set all these:

    git config --global fetch.all true
    git config --global pull.ff only
    git config --global merge.ff false